What is this manga called?

its about a poor girl who ends up falling in love with a wealthier guy who is the heir to some rich company. in a chapter she works as a cleaning woman for the company so she can buy a birthday present for him. the night of his birthday celebration the toilets back up so shes late to the party and the guy announces that shes his fiance. thats all i can remember. please help me remember the name of it!